Musannaf Ibn Abi Shayba 3:178

Abu Bakr narrated to us, he said: Ya'la narrated to us, he said: Al-A'mash narrated to us, from Ibrahim, from Al-Aswad, who said: When Umar would raise his head from the bowing (ruku'), he would say, "Allah hears those who praise Him," before he straightened his back.